Copper foil used in the manufacture of PCB's is typically produced on a rotating drum cathode machine partially immersed in a sulfuric acid/copper sulfate electrolyte, wherein an electric current is passed through the electrolyte from one or more anodes to the drum cathode to electrodeposit the copper foil on the surface of the cathode. The resulting foil had one side with a relatively smooth (shiny) surface, often referred to as the drum side, and the opposite side, which has a velvety (matte) surface, is often referred to as the electrolyte side. The microprofile of the electrolyte side of the raw foil is formed of micro-peaks and micro-valleys when observed under a microscope.
In the field of electrodeposited copper foil destined for printed circuits, "barrier layer" is a term to describe a metallic coating plated over a copper bond-enhancing treatment deposited on a bonding side of a copper raw, or base, foil.
Thus, the barrier layer forms the outer perimeter of the bonding treatment, and as such interfaces with polymeric substrates in the manufacture of copper clad laminates. The basic raw material for the manufacture of printed circuits is a laminate clad with copper foil which comprises of a thin copper foil firmly bonded to a polymeric, dielectric (insulating) substrate material. This "bonding" operation is carried in laminating plants and involves heating and cooling cycles. Sheets of copper foil are laid upon sheets of "prepreg" (e.g. glass fabric impregnated with epoxy resin). Both materials are placed in a hydraulic press, having pressing plates which are heated, while the two materials are pressed together (high psi). At selected temperatures, the resin liquefies and is forced, by the pressure, to flow into the micro-irregularities of the foil surface. This is followed with a second cycle, when both materials are cooled, while the pressure is maintained. The resin solidifies in the irregularities on the foil surface and both materials are firmly bonded together and are very difficult to pull apart. The "peel strength" between both materials is high, because the bonding side of the copper foil is provided with the bond-enhancing treatment. High peel strength is a characteristic of the highest importance since the mechanical support of the circuit elements, as well as the current carrying capability of PCB's, is provided by the copper foil - polymer joint. It is essential that the foil is bonded very tightly and securely to the laminate and also that such an adhesive joint can withstand all the manufacturing steps in PCB's fabrication without a decrease of the initial adhesion, which, moreover should remain constant through the service life of the PCB.
The bonding treatment, usually formed in two steps, typically includes a first dendritic copper deposit, followed by an encapsulating, or gilding, layer of copper, as disclosed in U.S. Pat. Nos. 3,857,681, 4,572,768 and 5,207,889, and is composed of copper, while the barrier layer deposited on the treatment is composed of zinc or brass, a zinc-nickel alloy, or another metal essentially chemically inert to the polymeric substance, which, in the process of lamination is in a semi-liquid, flowing state to effect the bonding to the treated side (surface) of the copper foil. The most common polymeric substrate used in the fabrication of printed circuits is a glass fabric impregnated with epoxy resin. Epoxy curing agents (catalysts, hardeners) belong usually to the class of organic derivatives of ammonia, are highly reactive chemically, and typically they are: amine complexes, e.g., tertiary amines, polyamines, aromatic polyamines, etc. Probably the most known epoxy curing agent is dicyandiamide NH.sub.2 C(HH)(NHCN).
Reactivity of copper with ammonium compounds (amines, amides) is well known, and explains the need for the barrier layer. The purpose of the barrier layer is to prevent direct copper resin contact.
If the bonding treatment composed of copper only (no barrier layer) is subjected to lamination with an epoxy substrate, the metallic copper reacts with the amino catalysts present in the resin. These reactions are particularly harmful to the quality of the printed circuits. They create moisture at the interface between the copper and the resin, causing harmful effects of measling and possibly de-lamination. In search of excellent dimensional stability, dielectric properties and long service life of PCB's, there has been a growing importance of new polymeric substrates that are superior in these respects to epoxy based materials.
Most of the new materials that are now commonly used in the manufacture of multilayer printed circuit board (MLB's) have glass transition temperatures (T.sub.g) substantially higher than epoxy. Fabrication of copper clad polyimide laminates require a 450.degree. F. laminating temperature compared with 325.degree. F. for epoxy, and a laminating time of 8 hours, compared with 3 hours for epoxy.
Polymers such as polyetherimides, polyamide-imide, polyphenylene sulfide have glass transition temperatures in excess of 480.degree. C., while Union Carbide's Udel (polysulfone) resin requires a laminating temperature of about 700.degree. F.
In addition, post baking operations are now practiced commonly, again in order to improve dimensional stability of printed circuit boards. By this practice copper clad laminates, e.g., epoxy based, are typically kept in ovens at temperatures of 380.degree. F. for 16 hours.
The idea is that any dimensional changes of copper clad laminate, shrinkage, warp, etc., will occur in the course of the post-bake. Thus the subsequent processing ir the fabrication of MLB's will produce boards that will be faultless in terms of registration and precision.
The practices described above impose very harsh conditions on the copper foil-polymer interface, conditions that threaten the forces of adhesion that join the two materials. Since the outer surface of the bonding treatment is a barrier layer, the harsh conditions of the interface particularly threaten, chemically, integrity and performance of
Barrier layers on polyimide-grade treatments have to withstand much higher laminating and post-bake temperatures, compared to the treatments destined for epoxy applications. High temperature at the metal-polymer interface can subject the metal surface to oxidation, with the attendant partial loss of adhesion. A well designed barrier layer will be self-protected, along with the underlying all-copper treatment, from heat oxidation and the loss of bond.
A good barrier layer should offer reasonable permanence and survival ability of the adherence under various conditions encountered during a PCB's manufacturing steps, as well as during PCB's service life. Successful stain proofing is synonymous with forming a stable film on the surface which promotes good adhesion and improves resistance to disbonding by various chemical environments, thus assuring durability to the foil-resin interface. During the manufacturing process of printed circuits, barrier layers are attached in a variety of ways. Narrow tracks of the foil are exposed to etching solutions, acids and hot water rinses, thermal shocks, etc. The chemicals and/or water tend to penetrate from the sides underneath the lines into the foil-polymer interface. If that happens, and it always happens to a degree, the real "functional" width of the track bonded to the polymer is diminished and thus the peel strength of the conductor line is diminished. It is the barrier layer's role to render the interface hermetic and guard it against ingress by the chemical environments. During the operations involving elevated temperatures, e.g., soldering, post-baking cycles, etc. the foil-polymer interface may suffer heat degradation, with the resulting peel strength loss. Here, the barrier layers offer overall shielding, capable of preventing undesirable reactions between the constituents of the resin system and the metal surface. The chemical attack on the circuit lines is a very real phenomenon and an easily demonstrable problem. If a narrow foil line is produced on the polymeric substrate by etching away un-masked foil, the resulting laminate with a narrow line is then subjected to a soak in hydrochloric acid, or boiling water, and then the line is peeled away from the substrate, one can see on both sides of the brass-colored matte side of the strip, well defined, lighter-colored margins where the chemicals have crept underneath and attacked the conductor. It is known that the chemical attack on the conductor lines is to a great extent dependent on the barrier layer.
It is known that the trends in MLB technology include the use of finer lines, increased density, smaller and more closely spaced holes, larger boards, higher speeds, surface mount, higher power dissipation, etc.
Manufacturing processes must continue to meet the above requirements, and the properties of the raw materials used must continually improve to satisfy these demands.
Perhaps two aspects of high quality sophisticated multilayer board design depend especially on the improved properties of copper foil. These aspects are a dimensional stability as features become smaller and boards bigger, and even more importantly, an improved copper adhesion as etched features become smaller and surface mounted components become more common.
Dimensional stability is the resistance of thin laminates to planar dimensional changes, either shrinkage or expansion, through processing. It is particularly critical in MLB's. The number value used to measure such stability is inches of movement per inch of board length in the X (warp or machine) and Y (fill) directions of the base reinforcement fabric.
It is an opinion of many technical people in the industry that the real issue is not really one of dimensional stability as, for instance, defined in MIL-P-13949 (which describes the changes in X and Y dimension in a piece of laminate after copper is etched off and it has been subjected to various specified thermal processes). The real problem, it is claimed, is the registration of inner layers.
Registration problems which occur during lamination of MLB's seem to defy meaningful correlation to the laminate characteristics called "dimensional stability". There seems little doubt that the facts which affect true position locations of internal lands of MLB's are related to stresses within the foil/laminate construction, which can affect registration later on. In the fabrication of MLB's copper foil is laminated (bonded to polymeric substrates) twice. First, thin, double-sided copper clad laminates are produced. These laminates are then subjected to image patterning and etching away of unwanted copper to produce the desired patterns of circuitry. Several layers of double-sided boards prepared in such a manner are stacked together, with sheets of prepreg (e.g., glass reinforced polymeric resin composites) inserted in between to separate dielectrically each inner board from the other. Such a stack of circuit boards and prepreg is then laminated together to form a monolithic multi-layer board. Later, thru-hole plating of copper is used to ensure the electrical interconnection between all layers of copper-track conductor lines. Increasing sophistication of multi-layer boards (e.g., narrower widths of track lines, narrower spacing between the lines, increasing number of inner layers) create an ever-increasing demand on the quality of copper foil used in the fabrication of MLB's (i.e., HTE foil), and particularly on the aspects of foil quality that depend on the properties of the barrier layer that protects the bonding side of the foil against dis-bonding (delamination) due to undercutting and thermal-degradation of the initial peel-strength that might result from high laminating temperatures and post-bake operations.
U.S. Pat. No. 3,857,681 (Yates et al.) is an excellent source of information pertaining to the bonding treatment technology. This patent discloses the concept of the sequential, plural layer bonding treatment technology which involves a succession of dendritic-powdery deposit, encapsulating deposit and the barrier layer. This patent discloses detailed process information and the parameters of the electrodeposition of all these layers. This patent discloses two parallel approaches to the practice of the barrier layer step of the overall treatment process. The barrier layer, according to the Yates et al. patent and the practice today by major foil manufacturers, consists of a thin layer of zinc alloy distributed uniformly over the micro-profile of an all-copper bonding treatment created in the course of the first two electrodeposition steps. This zinc-alloy layer, i.e., barrier layer, can be either plated "as is" or created by the heat accelerated diffusion of the metals. Some foil manufacturers electrodeposit brass, yellow zinc-copper alloy. Others electrodeposit zinc, a gray metal, in which case the foil delivered to the laminating plants has the bonding side characterized by the uniform gray color. Since the laminating process involves heat in the course of fabrication of copper clad laminates destined for PCB's, the zinc barrier layer alloys with the underlying all-copper bonding treatment by the process of heat-accelerated diffusion of metals in the solid state. As a result, a layer of yellow, chemically stable alpha brass is thus formed over the surface of all-copper surface. The Yates et al. patent discloses the alternative use of metals such as nickel, cobalt, chromium, cadmium, tin and bronze in the formation of barrier layers. Subsequently, major foil manufacturers have introduced the use of minor amounts of such metals as alloying elements co-deposited with zinc to improve the performance of the barrier layer with respect to high temperatures of lamination required in the fabrication of polyimide laminates, high temperature post-bakes, etc. Metals like nickel, cobalt, tin, etc., have been co-deposited with zinc to achieve these goals.
While barrier layers of zinc with nickel (by way of example) represent an improvement over an alloyed zinc or brass, in terms of resisting the loss of bond due to high laminating temperatures, undercutting, etc., none of the above barrier layers offers a truly complete immunity to the dis-bonding environments.
On the other hand the metals capable of forming the barrier layers that could be immune to high-temperature induced decay of peel strength did not find application in the practice of manufacturing copper foil for MLB's.
Although the prior art discloses the use of a cobalt barrier layer (U.S. Pat. No. 3,857,681 to Yates et al.) cobalt is not used in practice, because it is a ferromagnetic metal and if present in considerable quantity in a printed circuit it will interfere with surface mounted memory devices (ROM, EPROM, RAM and other magnetically stored devices). However, the prior knowledge of cobalt plating techniques does not enable plating a continuous layer of cobalt which is sufficiently thin to not lead to the magnetic problems, but at the same time is sufficiently continuous to form a barrier layer capable of good performance in terms of assuring "durability" of foil's bonding properties.